*All* UART drivers in TF-A are storing their base address as a uintptr_t
pointer in the first location of the UART specific driver data.
Since the base address is a pretty natural and generic data item, we
should integrate this into the generic console_t structure.

That will not only allow to remove a lot of seemingly UART specific data
structures, but also enables to simplify runtime choices between different
UARTs, since they can share the same pointer.

This patch just adds the new member, the existing data structures will
be handled on a per-UART base in follow-up patches.

Change-Id: I59ce49471ccc8f3b870f2cfd8a72ebfd0cb14d12
Signed-off-by: Andre Przywara <andre.przywara@arm.com>